Typically, a valve device is configured such that a valve seat and a valve body configured to be movable in a direction toward and away from the valve seat are included in a valve chest formed in a housing. The valve seat may be formed as a valve seat body separately from the housing and be provided in the valve chest. The valve device is connected to a passage (for example, a pipe) such that a fluid passage extends through the valve chest. The valve device is used to open and close the passage by the opening and closing of a valve and control the flow of the fluid.
FIG. 16 is a cross-sectional view showing the schematic configuration of a conventional valve device. A conventionally typical valve device 100 shown in FIG. 16 is configured such that a valve seat body 5 and a valve body 4 are included in a valve passage formed on a housing 21. The valve passage is constituted by a primary passage 32, a secondary passage 33, and a valve chest 31 provided between the primary passage 32 and the secondary passage 33. The valve seat body 5 having a thick cylindrical shape is provided at a portion of the valve chest 31, the portion being connected to the secondary passage 33. A sealing member 58 seals between the outer periphery of the valve seat body 5 and the inner wall of the valve chest 31. The inner periphery of the valve seat body 5 is a communication passage 50 configured to cause the valve chest 31 and the secondary passage 33 to communicate with each other. A valve seat portion 54 is formed at an opening edge of the communication passage 50, the opening edge being located on the primary passage 32 side. The valve body 4 is provided in the valve chest 31 so as to be able to move toward and away from the valve seat portion 54. In the valve device 100 configured as above, the valve is closed when the valve body 4 is seated on the valve seat portion 54, and the valve opens when the valve body 4 separates from the valve seat portion 54.
In the conventional valve device, normally, the housing and the valve body are made of metal and have high stiffness. In addition, known as the valve seat body configured separately from the housing are a valve seat body made of a synthetic resin (see PTL 1) and a valve seat body made of metal and having a valve seat portion to which a sheet member made of a soft material, such as a resin, is bonded (see PTL 2). As above, the airtightness of the valve is improved by configuring at least the valve seat portion of the valve seat body by using the resin.